This invention relates generally to the art of investment casting, and more specifically to ceramic shell molding techniques of investment casting wherein shell molds suitable for casting metal are prepared by building up layers of refractory material around disposable patterns that are subsequently removed from the molds.
Ceramic shell molds are prepared using patterns that are replicas of the parts to be cast and which may include the necessary gates and risers. The patterns are formed of any expendable materials, such as wax or a suitable synthetic resin or blends of wax and resin. These patterns are attached to a central support or sprue member to form what is known as a "set up" or "tree". The formation of a shell mold around the tree is generally accomplished by applying a refractory slurry coating of controlled viscosity followed by directional draining to coat the patterns completely. After draining excess slurry from the tree, the slurry coating is sanded or stuccoed while wet with coarser refractory materials. This layer is hardened, as by forced air drying at room temperature. The result is a layer of ceramic material having refractory particles embedded in the surface. After the first ceramic layer is sufficiently hard and dry, the steps of coating, draining, stuccoing and drying are repeated until a refractory shell having a sufficient thickness to resist the stresses occuring in subsequent operations has been built up around the tree. In a subsequent pattern removable operation, the tree, including the patterns, is removed from the shell mold and it is prepared for the casting operation.
The present invention is an improvement on the invention described and claimed in U.S. Pat. No. 3,424,227 issued Jan. 28, 1969 to C. H. Watts, et al. As disclosed in that patent, a plurality of mold shells are formed in a single operation using one tree. This is accomplished by providing the tree with one or more dividers extending around and projecting radially from the central support or sprue member between its ends. The refractory material used to form the mold shell around the tree is removed from the outer periphery of each divider so that the mold shell is separable into individual sections, each of which is usable to form a mold.
The preferred practice has been to remove the mold material from the dividers after each stucco application before the material has hardened. Prior to hardening, the mold material is soft and wet and can be easily removed by wiping. The disadvantage of this practice is that the wiping operation entails in-process time and requires a work station in the production line. An alternative practice is to remove the hardened mold material from the outside of the dividers by grinding or wire brushing. This can be done after the shell has been formed so as to eliminate special in-process steps during the mold making operation. However, the practice has definite disadvantages in that it usually requires more time and generates objectionable dust.
The development described in U.S. Pat. Nos. 3,812,898 and 3,871,440 issued May 28, 1974 and Mar. 18, 1975, respectively, to Robert A. Horton provides a method and apparatus of coating a plurality of trees or pattern assemblies in a single operation. In the preferred embodiment of that development, the individual trees or pattern assemblies are mounted on a fixture so that they do not individually rotate on their own axis. The arrangement usually makes it impractical to remove the soft, wet mold material from the divider before hardening, since the dividers are not readily accessible. As a result, it has been necessary to resort to the less desirable alternative of grinding or wire brushing the hard mold material from the peripheries of the dividers after the mold making operation has been completed.